1. Field of the Invention
The present invention refers to a method and an apparatus for converting a liquid flow into a gas flow and especially to a method and an apparatus for converting a liquid flow into a gas flow by means of dispersion.
2. Description of the Related Art
Various methods of converting a liquid flow into a gas flow are known. They are used e.g. in cases where liquid chemicals must be converted into the gaseous phase, e.g. when specific chemical deposition processes are carried out in the field of semiconductor technology.
The best known method for converting a liquid flow into the gaseous phase is the so-called bubbler method. This method comprises the steps of heating the liquid to be evaporated to a predetermined temperature and introducing a so-called carrier gas, e.g. nitrogen or oxygen, continuously below the surface of the liquid to be evaporated. The carrier gas bubbles rise in the liquid and are saturated with the vapour of the liquid. A mixture of vapour and carrier gas forms in a closed volume above the surface of the liquid, the mixture being discharged e.g. into a reactor connected to the volume and being then used in the above-mentioned chemical deposition processes. When an adequately low counterpressure (e.g. reactor pressure) and a high vapour pressure of the liquid are used, a vapour stream can also be produced without a carrier gas. Depending on the vapour pressure of the liquid, this, however, requires high temperatures of the liquid.
Although the bubbler method described hereinbefore is simple with regard to the apparatus used, the control of the vapour stream is very difficult. The mass flux of the vapour/gas mixture is determined by the temperature of the liquid, the pressure above the surface of the liquid and the carrier gas flow rate. In addition, the temperature of the carrier gas and the height of the column of liquid can influence the stability and the continuity of the vapour stream. In the case of this known method, major variations of the mass flux and even a complete interruption of the mass flux caused by gas cavities may occur. In general, it is not always avantageous to admix a carrier gas for technological reasons.
Another method is based on the principle of achieving an evaporation process which is referred to as flash evaporation. Flash evaporation processes are normally used for evaporating materials with different vapour pressures. The desired composition of the gas mixture is achieved in that small amounts of the starting material, in a defined composition, are evaporated completely. In this connection, a distinction is made between the introduction of the material, the method of distributing the material to be evaporated and the heating.
An essential drawback of this method is that it is complicated and difficult to achieve a uniform distribution of the liquid in the evaporator. In the flash evaporation method, the liquid is introduced in the evaporator by means of a mass flow controller or a dosing pump. The mass flow controller supplies liquid under a very low pressure, a circumstance which makes it more difficult to distribute the liquid uniformly over the evaporator surface which is a large surface in most cases. This may give rise to temperature inhomogeneities caused by different cooling processes, and these temperature inhomogeneities result in local condensation. The dosing pump is capable of producing a high pressure, but, if the liquid contains gas cavities, this will have the effect that the liquid flow is disturbed or interrupted completely.
A further method, which is disclosed in the European patent application EP-A-0559259, is based on the principle that a liquid to be evaporated is introduced in a mixing chamber and that a carrier gas flow is simultaneously introduced in the mixing chamber in close proximity to the liquid inlet nozzle. The carrier gas flow is introduced under a pressure which is so high that a liquid/gas mixture is already generated immediately behind the liquid inlet nozzle, the liquid/gas mixture being introduced in an evaporator chamber. This evaporator chamber can have heat supplied thereto so that the liquid will evaporate and a gas mixture will form which is finally discharged from the evaporator chamber.
Other known methods are based on the principle that a liquid to be evaporated is applied to a large, heated surface so as to achieve a good heat transfer and rapid evaporation.
The publication Applied Physics Letters, Vol. 56, No. 25, pages 2522 and 2523 describes a method of spraying in the interior of a pyrolysis reactor. A liquid is supplied to an ultrasonic nozzle by means of which the liquid is atomized. The atomized liquid is entrained by a stream of oxygen.
Patent Abstracts of Japan, Vol. 14, No. 77 corresponding to JP 1294525 concerns the spraying of a solution with metal components making use of an ultrasonic spray means. From this publication it can not be inferred whether the spraying of the liquid takes place here with or without a change of pressure or whether it is carried out with or without admixture of a medium.
Patent Abstracts of Japan, Vol. 6, No. 90 corresponding to JP-A-57022136 concerns the production of optical glass fibres. A glass-forming raw material liquid is introduced in a reservoir and atomized by means of a vibration energy provided by an ultrasonic vibrator, an inert gas introduced in the reservoir conveying the material to an outlet.